1. Field of the Invention
The invention relates to a video system with automatic gain control, including a variable gain video amplifier responsive, via a control circuit, to the output of a detector coupled to the signal output of the video amplifier. A video system of this kind, having automatic gain control via feedback, is used to obtain a video image of constant brightness, for example the mean or peak brightness in the event of a varying intensity of the video signal applied to the input of the amplifier, for example due to variation of illumination of or absorption in an object observed by means of the video system.
2. Description of the Related Art
A video system of this kind is known from DE-A 39 25 329. Therein, a circuit for a video system is described in which the output signal of a video camera is amplified by means of a controllable amplifier. The amplified signal charges a first capacitor via a system of switches and a rectifier. The charge across the first capacitor is transferred, via a further switch, to a second capacitor which is connected to a first input of a differential amplifier. It is thus achieved that the voltage across the second capacitor represents the current value of the brightness obtained for the brightness control. The second input of the differential amplifier receives a reference voltage. The output signal of the differential amplifier is used to control the gain of the video amplifier.
In the known video system, the gain of the video amplifier is varied by an amount which is proportional to the difference between the actual brightness and the reference value. When the mean brightness of the video signal presented to the video amplifier changes, to begin with the gain is then changed by a value which is proportional to the change of the amplitude of the video signal. It is only subsequently that via the feedback the gain is readjusted until the desired level of the output signal is obtained again. Consequently, the adjusted brightness is reached only in the course of time and, in the event of large level variations of the video signal, a damped oscillating level of the output signal may occur.